Propellant fuel containing magnesium aluminum alloy



April 27, 1965 G. J. MILLS ETAL 3,180,770

PROPELLANT FUEL CONTAINING MAGNESIUM ALUMINUM ALLOY Filed May 24, 1963 WEIGHT PER CENT MAGNESIUM IO 20 3O 4O 5O 60 7O 8O 90 700 l l l I O 2O 3O 4O 5O j A1 8O ATOMIC PER CENT MAGNESIUM PHASE DIAGRAM OF THE Mg-A1 SYSTEM GEORGE J. MILLS WARREN L. DOWLER DERCK A. GORDON, INVENTOR.

W WZJMM BY 523.91g 4w lVMM /06 United States Patent 3 180 770 PROPELLANT FUEL COblTAlNING MAGNESIUM ALUMINUM ALLOY George J. Mills, Santa Ana, Warren L. Dowler, Roseville, and Derek A. Gordon, Palos Verdes Estates, Caliil, assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed May 24, 1963, Ser. No. 284,776 2 Claims. (Cl. 149-19) This invention relates to metal fuels for rocket propellants and explosive compositions. More particularly the invention concerns aluminum-magnesium alloys as fuels in rocket propellant and explosive compositions.

To achieve greater thrust from reaction motors, it has become necessary to improve the propellants. I One means by which greater thrust has been achieved has been through the use of metals as fuels. Among the metals which have been used are aluminum, magnesium, boron, beryllium, and lithium. The manner in which metals have been used in both liquid and solid propellant systems is illustrated in the following US. patents: 2,771,739; 2,890,108; 2,- 926,613; 2,989,388. Moreover, metals have been used in explosive compositions as shown by US. Patent 2,589,532.

Some of the properties desired in a metal fuel are (1) high density, (2) high heat of oxidation, (3) low ignition temperature, (4) rapid and complete'combustion within the motor chamber, and (5) negligible reactivity with other propellant constitutents such as the binder and oxidizer. Both magnesium and aluminum are valuable fuels from the standpoint of high heat of oxidation, but neither alone satisfies all ofthe other requirements.

It has now been determined that alloys of magnesium and aluminum are excellent for use as fuels in rocket propellants and explosives. The alloys possess all the desired properties set forth hereinabove whereas neither aluminum nor magnesium alone is completely satisfactory.

The Mg-Al alloys possess higher density than magnesium alone and are morecompatible with other conventional propellant ingredients, such as polyurethane binders and ammonium perchlorate, than is magnesium. On the other hand the Mg-Al alloys can be ignited at lower temperatures and burn faster and more completely than aluminum. The alloys do not suffer from a serious lowering of their heats of combustion. Furthermore, the alloys are brittle and can be easily ground into powder, or they can be atomized. Moreover, alloying aluminum with magnesium permits the acceleration of the burning rate of the aluminum without having to resort to grinding the aluminum to ever smaller particles since the smaller the particle size the greater the fire hazard, the cost, and the contamination of the aluminum by the formation of the metal oxide coating. r

In view of the foregoing it is an object of the present invention to provide a Mg-Al alloy fuel which is more compatible with other conventional propellant and explosive ingredients than either metal alone.

Another object of the invention is to provide a Mg-Al alloy which has a low ignition temperature and burns rapidly and completely.

A further object of the instant invention is to provide Mg-Al alloys that can be easily ground into particles or atomized for use as a fuel in rocket propellants and explosives.

A still further object of the invention is to provide Mg-Al alloys which can be employed in relatively large particle sizes and thereby greatly reduce fire hazards and the deleterious effects of surface oxidation.

The manner in which these and other objects can be accomplished will become apparent from the detailed description presented hereinafter.

FIGURE 1 is a phase diagram of the Mg-Al system having superimposed thereon a broken line graph of the Patented Apr. 27, 1965 ice ignition temperature of the alloy as a function of the alloy composition. It is immediately apparent from FIGURE 1 that a wide range of alloy compositions exists for which the melting points are approximately 200 C. below those melting points of either aluminum or magnesium. Low melting points generally correspond to low ignition temperatures as reflected by the graph. Moreover, the low melting points of these compounds indicate that appreciable heats of formation are not involved, and, therefore, alloys containing these phases would not sufier from a serious lowering of their heats of combustion.

The actual preparation of the Mg-Al alloys to be used according to the invention is quite simple. All that is required is the heating together of the predetermined amounts of aluminum and magnesium until both metals melt. The melt is thoroughly, mixed by agitating the molten mass and allowed to solidify. It is customary in the formation of alloys to first melt the metal having the highest melting point. Since aluminum melts at 660.2 C. it will normally be melted first and the magnesium added to the aluminum melt. However, as magnesium melts at 651 C. the order of melting the metals is not critical since the melting points differ only by ten degrees. There is no danger of undue vaporization losses since magnesium does not boil until it is heated to 1103 C. Because the formation of oxides is detrimental to the alloy, it is preferred to conduct the alloying process in an inert atmosphere such as in argon or helium.

After the melt solidifies, the alloy can be ground into particles of any desired size for incorporation in propellant or explosive compositions. Generally, the Mg-Al alloy will be employed as a powder having an average particle size of 10 microns to 200 microns although smaller or larger particles can be used. However, one of the advantages of the Mg-Al alloys of the invention is that ignition and complete combustion can be achieved with out the use of extremely small particles. Therefore, the particles are preferably relatively large, for example 150 microns to 300 microns. The alloy particles normally comprise from 5% to 25% by weight of the total composition.

Table 1 below sets forth the physical properties of various Mg-Al alloys which were prepared to determine those TABLE I Some recorded properties of a number of alloys used Wt. Frac- Observed Density, PerMce)nt Duetility ture 1 Microstructure Z g./cm.

0 Ductile 2. 5 do a Matrix+ precipitate. 10 do a Matrix+some 2. 60

eutectic.

. do a Matrix-l-eutectic--. Britgle, tough do 2. 42

Brittle, hard do (1 do Divorced eutectic;

Ifflractures through do T Eutectoid 2. 22 Very Brittle I d Brittle, hard T Hard and ex- '1 tremely brittle d0 T Brittle, hard T 7 Single phase 2.09 do T 'y Matrix+eutectie Very Hard, but Eutectic 1. 99

brittle. Ductile, extremely 8+Eutectle matrix tough. 8O rln fln 1 100 Ductile 1. 74

1 I=intercrystalline; T=transerystalline. 1 For Laboratory-prepared specimens, as cast.

best suitable as metallic fuels. The samples Were prepared in 50 g. to 100 g. lots in the manner described above. The alloys comprising 20% to 70% by weight magnesium are brittle and easily lendthemselves togrinding.

One unexpected characteristic of the alloys studied is the abrupt increase in completeness and inthe rate of burning of aluminum in the range of by weight magnesium. Particles of aluminum, unless 10 microns or less in size, are difficult to ignite. Once ignited in an oxygen atmosphere these particles reach only a low temperature by self heating before being extinguished. With about 5% by weight magnesium the particles ignite easily and burn rapidly with a bright white heat characteristic of complete aluminum combustion.

While the alloy fuels of the invention will find use in many explosive and propellant compositions, they are particularly well suited for use in propellant compositions using polyurethane binders and ammonium perchlorate as an oxidizer. The polyurethane binders are a Well known class of polymers used extensively as binders in solid propellants. They are usually prepared from organic diisocyanates and polyhydroxy compounds. A very satisfactory polyurethane binder is prepared from polyoxypropylene glycol and toluene-2,4diisocyanate.

A major problem in using magnesium powder in propellant compositions comprising a polyurethane binder and ammonium perchlorate as an oxidizer is the porosity of the propellant grain as a result of the evolution of gas,

during the curing process. This gas evolution is attributed to reaction of the metal with polypropylene glycol and/ or traces of water in the propellant mix. The use of Mg-Al alloys in lieu of magnesium in these compositions reduces gas evolution until it is negligible.

Since the rate of reaction of magnesium with water is more rapid than with polypropylene glycol, tests conducted with water are easier to run and the results more easily analyzed. Twenty tests were conducted with Mg-Al alloys ranging from 0% to 100% by Weight magnesium and having an average particle size of 150 mesh to 325 mesh. The evolution of hydrogen as a function of time was used to measure the results. The test results can be summarized as follows:

(1) The intermediate compound phases in the Mg-Al system between 38% and 63% by weight magnesium form protective oxides which decrease sharply the evolution of hydrogen.

(2) The rate of hydrogen evolution with pure magnesium is high; however, the addition of aluminum causes a marked increase in the rate which reaches a maximum at about 25% by weight aluminum. The addition of more aluminum, the rate decreases because an unreactive intermediate compound phase which precipitates by eutectic reaction tends to predominate in an amount over the reactive solid solution phase. The eutectic composition occurs at about 30% by weight aluminum.

(3) The rate of reaction of pure aluminum is negligible; however, the addition of magnesium increases the rate which is at a maximum at about to by weight magnesium. With the further addition of magnesium, the rate of reaction decreases because of the precipitation of an unreactive intermetallic compound phase by a eutecliquid fuels.

i tic reaction. The eutectic reaction occurs at about 37% by weight magnesium.

In accordance with the foregoing, it is seen that for use in propellant compositions using polyurethane binders and ammonium perchlorate as an oxidizer, the alloys comprising 38% to 63% by weight magnesium and less than 15% by weight magnesium are preferred. However, alloys containing less than about 5% by weight magnesiumare not noticeably superior in combustion performance to pure aluminum.

While the Mg-Al alloys of the invention are very useful in propellant compositions using polyurethanes as binders, especially those using ammonium perchlorate as an oxidizer, the superiority in combustion performance of the alloys makes them useful as fuels in any combustible compositions. For example, the alloys may be employed with propellants or explosives using as binders gelatinized nitrocellulose, polysulfides, polybutadiene-acrylic acid copolymers, nylon, and asphalt. In lieu of ammonium perchlorate, any conventional oxidizer such as ammonium nitrate, sodium nitrate, potassium nitrate, sodium perchlorate, or potassium perchlorate can be employed. Moreover, the alloys can be utilized as a solid suspension in Broadly stated, the alloys of the invention are useful as fuels in gas-generating compositions which produce large quantities of gases upon combustion of metallic fuels. 7

It is to be understood that when the Mg-Al alloys of the i invention are used as fuels with binders other than the polyurethanes, the constitution of the alloys can range from 5% by weight magnesium and 95% by Weight aluminum to about by weight magnesium and 30% by weight aluminum. However, due to density and other physical consideration, the alloys having from 20% to 70% by weight magnesium will normally be employed with those containing from 20% to 50% by weight magnesium being preferable. In the range of 20% to 50% by weight magnesium, the alloys having acceptable density, are brittle enough for easy grinding, and possess superior combustion properties.

The above detailed description is for the purpose of exemplification of the invention and no undue limitation should be attributed thereto except as reflected by the appended claims.

We claim:

1. A solid rocket propellant composition consisting essentially of a cured intimate admixture of a polyurethane binder, metallic fuel particles, and an oxidizer therefor, said fuel particles being an aluminum-magnesium alloy consisting essentially of about 38% to about 63% by weight magnesium the remainder of said alloy being aluminum.

2. A composition according to claim 1 wherein said oxidizer is ammonium perchlorate.

References Cited by the Examiner UNITED STATES PATENTS 3,028,808 4/62 Porter et al. 149-42 X 3,044,911 7/62 Fritzlen 149-37 CARL D. QUARFORTH, Primary Examiner. 

1. A SOLID ROCKET PROPELLANT COMPOSITION CONSISTING ESSENTIALLY OF A CURED INTIMATE ADMIXTURE OF A POLYURETHANE BINDER, METALLIC FUEL PARTICLES, AND AN OXIDIZER THEREFOR, SAID FUEL PARTICLES BEING AN ALUMINUM-MAGNESIUM ALLOY CONSISTING ESSENTIALLY OF ABOUT 38% TO ABOUT 63% BY WEIGHT MAGNESIUM THE REMAINDER OF SAID ALLOY BEING ALUMINUM. 